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pianotuna

Regina, SK, Canada

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Posted: 06/28/22 08:56am Link  |  Quote  |  Print  |  Notify Moderator

Hi All,

Regarding the Renogy 20A DC to DC Battery Charger 12V, the unit puts out 20 amps, but draws 30 amps when doing so. This is the result of bench testing.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

CA Traveler

The Western States

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Posted: 06/28/22 09:04am Link  |  Quote  |  Print  |  Notify Moderator

Not very efficient. [emoticon]

Wonder if it varies by voltage or amps.

* This post was edited 06/28/22 09:19am by CA Traveler *


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BFL13

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Posted: 06/28/22 09:55am Link  |  Quote  |  Print  |  Notify Moderator

I got mine to do better with a higher input voltage. What was the input voltage on yours when input amps was 30?

Of course you have to have input watts higher than output watts, so what was the output voltage when it was doing 20a?

I run my output set to 14.7v and it does the 20 amps out steady. I had to improve the wiring from engine battery to Renogy input to reduce voltage drop and so keep input up enough.


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Gdetrailer

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Posted: 06/28/22 01:19pm Link  |  Quote  |  Print  |  Notify Moderator

CA Traveler wrote:

Not very efficient. [emoticon]

Wonder if it varies by voltage or amps.


Correct.

Each time you do some sort of "conversion" there is some efficiency loss which results in some heat.

Additionally going from a lower voltage to a higher voltage means it takes more current on the input than the current you get on the output but if you check the watts they will be nearly identical minus of course the conversion loss.

For instance.. Say you are converting 12.0V to 14.0V..

12.0V at 30A is 360W.

360W at 14.0V is 25.7A not including the efficiency loss of the switching power supply..

So depending on the start voltage of the OPs test and what his target output voltage it is very possible that his input current can be 30A at the lower voltage but yet only yield 20A at his higher target voltage but yet still get the same watts..

This is no different than what the Power Company does to distribute AC on the grid.. They take the generated voltage, run through a high voltage transformer which boosts the voltage and that drops the current allowing thinner wire to be used. Then at your house run it through another transformer to drop to a more usable lower voltage at higher current for your home.

pianotuna

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Posted: 06/28/22 01:20pm Link  |  Quote  |  Print  |  Notify Moderator

Hi All,

I did not do the bench test--my cousin did, so I don't have answers to your questions. I thought it might be useful to others.

I'll be "feeding" the unit with #8 wire.

I have a choice of powering from the alternator, or powering from the starter battery (which is of course charged by the alternator), but only with the ignition key turned on. Doing the battery route would eliminate surge on the alternator, so I'm leaning in that direction.

I'll have the ability to switch the dc to DC off.

I'll still have a "poor man's boost ability, too.

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Posted: 06/28/22 01:57pm Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer - Thanks for the reminder of watts.

BFL13

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Posted: 06/28/22 03:05pm Link  |  Quote  |  Print  |  Notify Moderator

#8 might not be enough. I had to change my input neg path to use the frame (as a really fat wire) to get my voltage drop small enough to keep input voltage in the 13s. You can tell by if it can't hold that 20 amps output, that you need more input (ie, less voltage drop)

I can't remember the unit's efficiency, but for instance:

14.7 x 20 = 294w and 85% efficiency (eg) needs 346w input.

If input voltage is 13v after v drop from engine batt, 346/13 = 26.6 amps. If you could keep input voltage at 14, then 346/14 = 25 amps

You are supposed to get input from the engine battery, not from the alternator. You need a common ground like it shows in the manual. With a Class C that would be the frame

The unit turns on and off using the little wire that connects to pos input. I have a switch on that little wire in my set-up, and just ran that wire over to the pos input terminal.

You must not also have the OEM alternator charging to house connected like in a MH, or you will add to the amps demand on the alternator, which the DC-DC is supposed to limit.

Pick the nearest charging profile on the Renogy to what your batt spec is (14.7 on mine) and then set your solar controller to match that voltage so they add their amps best while driving down the road.

wa8yxm

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Posted: 06/28/22 04:22pm Link  |  Quote  |  Print  |  Notify Moderator

Watts out = watts in plus overhead and conversion loss.

A DC=DC converter usually is an inverted/charger paired. Power in is chopped into Square wave A/C and then processed. This is about 90% efficient going in and another 10% is lost going out that's best case. Few are that good.

Now.. some interesting things.. Because it's not converting to 60Hz AC but usually a much higher frequency. the processing is easier and if it uses a transformer that's much lighter.. But basically you have an inverter and a converter in one box.


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pianotuna

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Posted: 06/28/22 04:25pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13,

#8 has a voltage drop of 2.46% at 10 feet (more than I need). It can carry 55 amps.

The whole idea of the dc to DC is to boost the voltage going into the battery bank. Therefore voltage drop to the charger doesn't matter--so long as the ampacity of the wire size is not exceeded.

On the Output side I might have about a 4 foot run. Voltage drop about 0.76% with #8 wire at 20 amps. Pretty much I think that can be ignored.

I must look up the charging voltage on the telcom jars that have served me so very well.

Thanks for the thought provoking comments.

BFL13

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Posted: 06/28/22 05:24pm Link  |  Quote  |  Print  |  Notify Moderator

"The whole idea of the dc to DC is to boost the voltage going into the battery bank. Therefore voltage drop to the charger doesn't matter--so long as the ampacity of the wire size is not exceeded."

Yes it does matter.

The unit is "buck/boost" and the output voltage will be higher than the input's in many situations so it has to boost. The voltage drop to the charger is vital to keeping the input watts high enough to keep the watts up. It also affects the amps draw on the alternator, which you want to keep as low as possible.

I had a problem where my output amps fell off from 20 and it turned out the input voltage was too low. The unit has a boost spec from about 8 volts but not quite so in real life. Anyway, you improve your input voltage "somehow" until the unit can sustain its full output amps at the chosen output voltage. You cannot control the input amps directly.

I am using the thin and long positive wire in the 7-pin as input so to get the whole circuit better, I had to swap over to using the frame instead of the neg 7-pin long thin wire. If that had not worked, I would have had to run a fatter positive wire too.

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